Network management device, network management system and network management method

ABSTRACT

A network management device determines at least one of that an individual error occurs in a target terminal device when a difference between (i) a response time for the target terminal device and (ii) an average response time that is an average value of the response time for each of two or more terminal devices belonging to a group to which the target terminal device belongs is larger than a first predetermined value, and that a group error occurs in a target group when a difference between (i) a current average response time that is an average value of the response time and (ii) a previous average response time that is an average value of a previous response time for two or more terminal devices belonging to the target group is larger than a second predetermined value, the previous response time being immediately prior to the response time.

BACKGROUND 1. Technical Field

The present disclosure relates to a network management device, a networkmanagement system, and a network management method. The presentdisclosure particularly relates to a network management device, anetwork management system, and a network management method that monitorstates of a predetermined network and a plurality of terminal devicesconnected to the network.

2. Description of the Related Art

Patent Literature (PTL) 1 provides a network failure detection systemand a network failure detection device capable of setting a thresholdvalue of an appropriate round trip time (hereinafter, referred to asRTT) in consideration of a propagation delay. The network failuredetection device according to PTL 1 groups monitoring target deviceshaving similar RTT values based on the RTT values of the monitoringtarget devices acquired at a usual time (normal time). The networkfailure detection device generates a distribution of the RTT values foreach group, and determines a threshold value of the RTT of each groupbased on the generated distribution of the RTT values.

PTL 1 is Unexamined Japanese Patent Publication No. 2014-68283.

SUMMARY

A network management system according to the prior art transmits amessage to each of a plurality of terminal devices connected to onenetwork, and compares an RTT value that is a response time of a responsemessage with a predetermined threshold value. When the response time islonger than the predetermined threshold value, a failure is detected inthe network and the terminal devices. However, in a case where a failureis detected only based on the response time of each terminal device, itis difficult to determine whether the delay in the response time iscaused by a failure occurring in the network or by a failure occurringin one of the plurality of terminal devices. In addition, it isnecessary to manually or automatically determine the predeterminedthreshold value for each terminal device based on statisticalinformation, and thus a preliminary preparation requires additionalexpense.

The present disclosure provides a network management device, a networkmanagement system, and a network management method capable ofspecifying, when a connection failure occurs between the networkmanagement device and a terminal device, whether the failure is derivedfrom a network or from the terminal device.

A network management device according to the present disclosure dividesa plurality of terminal devices connected to a network into a pluralityof groups, transmits transmission data to the plurality of terminaldevices, and receives response data corresponding to the transmissiondata to acquire connection states of the plurality of terminal devices.The network management device includes at least one of an individualerror determiner configured to determine that an individual error occursin a target terminal device belonging to a group among the plurality ofgroups when a difference between (i) a response time from a transmissiontime of transmission data to a reception time of the response data forthe target terminal device and (ii) an average response time that is anaverage value of the response time for each of two or more terminaldevices belonging to the group is larger than a first predeterminedvalue, and a group error determiner configured to determine that a grouperror occurs in a target group to which two or more terminal devicesbelongs among the plurality of groups when a difference between (i) acurrent average response time that is an average value of the responsetime for each of the two or more terminal devices and (ii) a previousaverage response time that is an average value of a previous responsetime for each of the two or more terminal devices is larger than asecond predetermined value, the previous response time being immediatelyprior to the response time.

A network management method according to the present disclosure is amethod for dividing a plurality of terminal devices connected to anetwork into a plurality of groups, transmitting transmission data tothe plurality of terminal devices, and receiving response datacorresponding to the transmission data to acquire connection states ofthe plurality of terminal devices. The network management methodincludes measuring, for each of the plurality of terminal devices, aresponse time from a transmission time of transmission data to areception time of response data, and at least one of determining that anindividual error occurs in a terminal device belonging to a group amonga plurality of groups when a difference between (i) the response timefor the terminal device and (ii) an average response time that is anaverage value of the response time for each of two or more terminaldevices belonging to the group is larger than a first predeterminedvalue, and determining that a group error occurs in a target group towhich two or more terminal devices belongs among the plurality of groupswhen a difference between (i) a current average response time that is anaverage value of the response time for each of the two or more terminaldevices and (ii) a previous average response time that is an averagevalue of a previous response time for each of the two or more terminaldevices is larger than a second predetermined value, the previousresponse time being immediately prior to the response time.

According to the network management device or the like of the presentdisclosure, when a connection failure with a terminal device occurs, itis possible to specify whether the failure is derived from the networkor from the terminal device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration example of anetwork management system according to a first exemplary embodiment.

FIG. 2A is a block diagram illustrating a detailed configuration exampleof network management device 804 in FIG. 1 .

FIG. 2B is a flowchart illustrating an example of control processing ofnetwork management device 804 in FIG. 1 .

FIG. 3 is a block diagram illustrating a detailed configuration exampleof packet analyzer 102 in FIG. 2A.

FIG. 4 is a block diagram illustrating a detailed configuration exampleof transmission data setting unit 103 in FIG. 2A.

FIG. 5 is a block diagram illustrating a detailed configuration exampleof group determiner 105 in FIG. 2A.

FIG. 6 is a block diagram illustrating a detailed configuration exampleof group response analyzer 106 in FIG. 2A.

FIG. 7 is a block diagram illustrating a detailed configuration exampleof communication state analyzer 107 in FIG. 2A.

FIG. 8 is a block diagram illustrating a detailed configuration exampleof error determiner 109 in FIG. 2A.

FIG. 9A is a timing chart illustrating an operation example of messagetransmission and reception in the network management system of FIG. 1 .

FIG. 9B is a diagram illustrating a table of an example of packetinformation analyzed by packet analyzer 102 in the network managementsystem of FIG. 1 .

FIG. 10A is a diagram illustrating a table of response times andprevious response times of terminal devices 801 belonging to group A inthe network management system of FIG. 1 .

FIG. 10B is a diagram illustrating a table of response times andprevious response times of terminal devices 801 belonging to group B inthe network management system of FIG. 1 .

FIG. 11A is a diagram for describing an operation of error determinationprocessing (individual delay determination) in the network managementsystem of FIG. 1 .

FIG. 11B is a diagram for describing an operation of error determinationprocessing (group delay determination) in the network management systemof FIG. 1 .

FIG. 12 is a flowchart illustrating an operation example of errordetermination processing in network management device 804.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments will be described in detail withreference to the drawings as appropriate. However, detailed descriptionmore than necessary might be sometimes omitted. For example, thedetailed description of already well-known matters and the overlappeddescription of a configuration substantially identical to thealready-described one may be omitted. This is to avoid an unnecessarilyredundant description below and to facilitate understanding of thoseskilled in the art.

Note that the inventor provides the attached drawings and the followingdescription to help those skilled in the art fully understand thepresent disclosure, and does not intend to limit the subject matterdescribed in the appended claims to these.

First Exemplary Embodiment

A first exemplary embodiment will be described below with reference toFIGS. 1 to 12 .

[1-1. Configuration]

FIG. 1 is a block diagram illustrating a configuration example of anetwork management system according to the first exemplary embodiment.In FIG. 1 , the network management system includes a plurality ofterminal devices 801, two router devices 802A, 802B, networks 803A,803B, and network management device 804.

In FIG. 1 , the plurality of terminal devices 801 are, for example,terminal devices such as a projector, a display, a printer, and ascanner. When receiving a request message, the plurality of terminaldevices 801 returns a response message. In FIG. 1 , a plurality of nterminal devices 801 connected to network 803A are denoted by A1 to An,and a plurality of m terminal devices 801 connected to network 803B aredenoted by B1 to Bm.

Router devices 802A, 802B are communication devices that managecommunication paths on networks 803A, 803B, respectively. Terminaldevices A1 to An are communicably connected to network management device804 via network 803A. Terminal devices B1 to Bm are communicablyconnected to network management device 804 via network 803B.

Network management device 804, which is, for example, a terminal devicesuch as a computer, executes a program stored in an internal or externalstorage medium to manage a communication state between the plurality ofterminal devices 801 and network management device 804 connected tonetworks 803A, 803B and present the communication state to a user (notillustrated). Abnormal portion 805 where an abnormality has occurred inthe communication state is present in network 803B, and a greater delayoccurs in a part of the communication path between terminal devices B1to Bm and network management device 804 than in the case of a normaloperation without abnormal portion 805.

FIG. 2A is a block diagram illustrating a detailed configuration exampleof network management device 804 of FIG. 1 . In FIG. 2A, terminal devicemanagement unit 104 stores terminal device information for managing theplurality of terminal devices 801 connected to the network by dividingthe terminal devices into a plurality of groups. The terminal deviceinformation includes, for example, address information such as localInternet Protocol (IP) addresses of terminal devices 801.

In FIG. 2A, transmission data setting unit 103 sets transmission datafor acquiring communication states of terminal devices 801 based on theterminal device information of terminal device management unit 104.Transmission and reception unit 101 transmits the transmission data setby transmission data setting unit 103 to the terminal devices 801 andreceives response data from terminal devices 801. Packet analyzer 102associates the response data received by transmission and reception unit101 with the transmission data set by transmission data setting unit103, measures the response time from transmission to reception, and thenoutputs a set of the terminal device information and the response datato group determiner 105.

Group determiner 105 determines whether terminal device 801corresponding to the response data belongs to the group, based on theinformation stored in terminal device management unit 104. When terminaldevice 801 belongs to the group, group determiner 105 further adds groupinformation to the terminal device information and the response data,and outputs the data to group response analyzer 106. When terminaldevice 801 does not belong to the group, group determiner 105 outputsthe input terminal device information and response data, as they are, tocommunication state analyzer 107. Group response analyzer 106 calculatesan average value of the response times of the terminal devices 801belonging to the group to which terminal device 801 belongs, and furthercalculates a time difference between the response time of terminaldevice 801 and the average response time of the group.

Reference time setting unit 108 sets a reference response time of eachterminal device 801 and a reference average response time of each groupas reference values of the response time of terminal device 801 at thenormal time and of the average response time of the group. Communicationstate analyzer 107 compares the reference response time set by referencetime setting unit 108 with the response data of terminal device 801, andanalyzes the communication state based on a delay amount from thecompared result. Similarly, reference time setting unit 108 analyzes thecommunication state by comparing the reference average response time setby reference time setting unit 108 with the average response time of thegroup. Error determiner 109 sets determination information for notifyinga user as a warning or an error, based on the analysis result ofcommunication state analyzer 107. Display unit 110 displays an obstaclefactor to the user based on the determination result of error determiner109.

FIG. 2B is a flowchart illustrating control processing of networkmanagement device 804. An operation of network management device 804will be described later.

Hereinafter, a detailed configuration of each component of the networkmanagement system of FIG. 1 will be described with reference to FIGS. 3to 8 .

FIG. 3 is a block diagram illustrating a detailed configuration exampleof packet analyzer 102 in FIG. 2A. In FIG. 3 , packet analyzer 102includes reception data storage unit 201, transmission data storage unit202, time information output unit 203, and reception data shaping unit204.

In FIG. 3 , time information output unit 203 outputs time informationindicating a current time to reception data storage unit 201 andtransmission data storage unit 202. Transmission data storage unit 202adds time information of the time when the transmission data is input(the time when the transmission data is transmitted to terminal device801) to the transmission data input from transmission data setting unit103 and saves the transmission data. Similarly, reception data storageunit 201 adds information of the time when the response data is input(the time when the response message is received from terminal device801) to the response data received from terminal device 801 viatransmission and reception unit 101, and saves the response data.Reception data shaping unit 204 reads the data stored in reception datastorage unit 201 and transmission data storage unit 202, andcollectively outputs the data to group determiner 105.

FIG. 4 is a block diagram illustrating a detailed configuration exampleof transmission data setting unit 103 in FIG. 2A. In FIG. 4 ,transmission data setting unit 103 includes communication commandmanagement unit 301, transmission data creation unit 302, andtransmission address setting unit 303.

In FIG. 4 , communication command management unit 301 stores a group ofvarious commands for transmitting and receiving information to and fromterminal device 801. Transmission address setting unit 303 sets anaddress (IP address) of terminal device 801 as a transmissiondestination based on the information of each terminal device 801 inputfrom terminal device management unit 104, and outputs the address to thetransmission data creation unit 302. Transmission data creation unit 302creates a request message to be transmitted to terminal device 801 usingthe address set by transmission address setting unit 303 and the commandgroup stored in communication command management unit 301. Transmissiondata creation unit 302 then transmits the request message to terminaldevice 801 via transmission and reception unit 101, and outputs therequest message to packet analyzer 102. As for a transmitted packet anda received response message, packet analyzer 102 associates the receivedresponse message with terminal device 801 that has transmitted thereceived response message.

FIG. 5 is a block diagram illustrating a detailed configuration exampleof group determiner 105 in FIG. 2A. In FIG. 5 , packet informationextractor 401 extracts association information about association betweenthe response data and each terminal device 801 that has transmitted theresponse data, and the information of the response time taken from thetransmission to the reception from each of the input response data, andoutputs the extracted information to determiner 402. Determiner 402determines whether each terminal device 801 belongs to any group, basedon the association information from group management unit 403. When eachterminal device 801 belongs to any group, determiner 402 outputs theextracted association information and response time information to groupsetting unit 404. Group setting unit 404 adds information on the groupto the association information and response time information, andoutputs the information to group response analyzer 106. When eachterminal device 801 does not belong to any group, group setting unit 404outputs the association information and the response time information tocommunication state analyzer 107.

FIG. 6 is a block diagram illustrating a detailed configuration exampleof group response analyzer 106 in FIG. 2A. In FIG. 6 , group responseanalyzer 106 includes time information extractor 501, response timedetector 502, previous value storage unit 503, and packet responseanalyzer 504.

In FIG. 6 , time information extractor 501 extracts time informationfrom the information input from group determiner 105. Response timedetector 502 calculates the response time of terminal device 801 basedon the time information, and outputs the response time to previous valuestorage unit 503 and packet response analyzer 504. Previous valuestorage unit 503 temporarily stores the response time input fromresponse time detector 502 at a previous time, and outputs the storedresponse time to packet response analyzer 504 as necessary. Packetresponse analyzer 504 obtains a difference between the response time atthe current time and the response time at the previous time, furthercalculates an average response time of the group, and outputs theresponse time information to communication state analyzer 107.

FIG. 7 is a block diagram illustrating a detailed configuration exampleof communication state analyzer 107 in FIG. 2A. In FIG. 7 ,communication state analyzer 107 includes reception data analyzer 601,delay amount analyzer 602, previous value comparator 603, andcommunication state setting unit 604.

In FIG. 7 , reception data analyzer 601 receives information on theresponse time of terminal device 801 from group response analyzer 106when terminal device 801 belongs to the group, or from group determiner105 when terminal device 801 does not belong to the group. Receptiondata analyzer 601 acquires a current state of terminal device 801 fromthe input information and outputs the current state to communicationstate setting unit 604. With respect to terminal device 801 that doesnot belong to the group, reception data analyzer 601 specifies a stateof connection between terminal device 801 and the network based on theinput terminal device information.

FIG. 8 is a block diagram illustrating a detailed configuration exampleof error determiner 109 in FIG. 2A. In FIG. 8 , error determiner 109includes individual error determiner 701, threshold value setting unit702, and group error determiner 703. In FIG. 8 , threshold value settingunit 702 sets a threshold value of the delay amount of the response timeof terminal device 801. Individual error determiner 701 determineswhether the delay amount of the response time of terminal device 801 islonger than the threshold value response time from threshold valuesetting unit 702. Group error determiner 703 determines thecommunication state in units of groups. The threshold value may be avalue empirically input by the user, or may be automatically determinedbased on a variation range of the delay time up to the present.

[1-2. Operation]

An operation of the network management system configured as describedabove will be described below with reference to FIGS. 2B and 9A to 12 .

FIG. 2B is a flowchart illustrating an operation example of controlprocessing in network management device 804 of FIG. 1 . In FIG. 2B,network management device 804 performs steps S100, S200, S300, S400,S500, and S1200. In step S100, transmission data setting unit 103 ofnetwork management device 804 sets transmission data to be transmittedto each terminal device 801.

In FIG. 2B, in step S200, network management device 804 transmits thetransmission data set in step S100 to each terminal device 801, andwaits until receiving response data from each terminal device 801. Insubsequent step S300, packet analyzer 102, group determiner 105, groupresponse analyzer 106, and communication state analyzer 107 analyze thereceived response data to acquire various pieces of information such asa response time and an average response time of a group.

In subsequent step S400, a determination is made whether terminal device801 belongs to any group. When terminal device 801 belongs to any group,the processing proceeds to step S500. When terminal device 801 does notbelong to any group, the processing proceeds to the error determinationprocessing in step S1200. In step S500, group response analyzer 106obtains a difference between the response time of terminal device 801belonging to a certain group and the average response time of the groupto which terminal device 801 belongs. Finally, in step S1200, errordetermination processing described in detail with reference to FIG. 12is executed.

FIG. 9A is a timing chart illustrating an operation example of datatransmission and reception processing in the network management systemof FIG. 1 . FIG. 9A illustrates transmission and reception of databetween network management device 804 and terminal devices A1, B1 instep S200 in FIG. 2B. Here, network management device 804 simultaneouslytransmits transmission data A1 s, B1 s prompting terminal devices 801 torespond to terminal devices A1 and B1, respectively. Thereafter, networkmanagement device 804 receives response data A1 r from terminal deviceA1 after time tA1 from the completion of transmission of transmissiondata A1 s, and receives response data B1 r after time tB1 from thecompletion of transmission of transmission data B1 s. Times tA1, tB1between the completion of transmission of transmission data A1 s, B1 s,and the completion of reception of response data A1 r, B1 r are referredto as response times of terminal devices A1, B1, respectively.

FIG. 9B is a diagram illustrating a table of an example of informationanalyzed by packet analyzer 102. As is clear from FIG. 9B, in packetanalyzer 102, the response time until reception of response data A1 r isrepresented by “tA1”, and the response time until reception of responsedata B1 r is represented by “tB1”.

FIGS. 10A and 10B are diagrams illustrating response time data outputfrom group response analyzer 106 to communication state analyzer 107.FIGS. 10A and 10B correspond to group A and group B, respectively. As isclear from FIG. 10A, terminal devices A1 to An are included in group A,and the response times of respective response data A1 r to Anr havechanged from previous response times tA1 p to tAnp into current responsetimes tA1 to tAn. Similarly, FIG. 10B illustrates that the responsetimes of response data B1 r to Bmr of terminal devices B1 to Bmbelonging to group B have changed from previous response times tB1 p totBmp into current response times tB1 to tBm.

FIG. 11A is a diagram illustrating a table of a response state ofterminal device 801 calculated in step S500 of FIG. 2B. In FIG. 11A,terminal devices A1 to An, which are included in group A, transmitsresponse data A1 r to Anr, respectively, to network management device804. Further, an average value of response times tA1 to tAn of terminaldevices A1 to An is average response time tAavg, and differences betweenresponse times tA1 to tAn and average response time tAavg arerepresented by differential response times ΔtA1 to ΔtAn, respectively.That is, the following equation holds.ΔtAi=tAi−tAavg (i=1,2, . . . n)Furthermore, it is assumed that threshold value α has been set bythreshold value setting unit 702 of error determiner 109. Results ofdetermining whether the differential response times are smaller thanthreshold value α by individual error determiner 701 of error determiner109 are shown as individual delay determination results (in FIG. 11A,“∘” when YES, and “x” when NO).

For example, in the case of FIG. 11A, since differential response timeΔtA1 is smaller than α, the delay determination indicates ∘, but sincedifferential response time ΔtA2 is larger than or equal to α, the delaydetermination indicates x. Similar individual delay determinationresults are determined for all the groups.

FIG. 11B is a diagram illustrating another table of the response stateof terminal device 801 calculated in step S500 of FIG. 2B. In FIG. 11B,an average response time (corresponding to the current average responsetime) of group B at current time t is represented by “gB (t)”. Inaddition, “gB(t−1)” that is the average response time at previous timet−1 is saved in previous value storage unit 503 of group responseanalyzer 106, and the difference ΔgB(t) of the current average responsetime from the previous average response time (corresponding to theprevious average response time) is obtained. That is, the followingequation holds.ΔgB(t)=gB(t)−gB(t−1)

Further, assuming that threshold value β is set by threshold valuesetting unit 702 of error determiner 109, a result of determiningwhether difference ΔgB(t) is smaller than threshold value β by grouperror determiner 703 of error determiner 109 is indicated by “∘” whenYES and by “x” when NO in FIG. 11B as a group delay determinationresult.

For example, in the case of FIG. 11B, since differences ΔgB(t−1) toΔgB(t−n) are smaller than threshold value β, the delay determinationindicates ∘, but since difference ΔgB(t) is larger than or equal tothreshold value β, the delay determination indicates x. A similar groupdelay determination result is obtained for each of all the groups. Errordetermination processing is executed in next step S1200 based on theindividual delay determination results and the group delay determinationresults.

FIG. 12 is a flowchart illustrating an operation example of the errordetermination processing in network management device 804. In FIG. 12 ,the error determination processing (step S1200) includes steps S1201 toS1207.

In FIG. 12 , first, in step S1201, network management device 804acquires the group delay determination results and the individual delaydetermination results for all the groups and all terminal devices 801.Next, in step S1202, network management device 804 determines whetherthe acquired group delay determination result indicates error (x). WhenYES, the processing proceeds to step S1204A, and when NO, the processingproceeds to step S1203.

In step S1204A, since the group delay determination result indicateserror, it is found that a delay has occurred in the entire group (groupB in the example of FIG. 11B). Therefore, a more detailed cause isanalyzed. For example, network management device 804 detects aphenomenon such that loss of the response data occurs and the responsedata is received but retry of transmission and reception is performed,and determines a more detailed cause such as poor connection status withthe network or a failure in authentication. After step S1204A, the errordetermination processing proceeds to step S1207, and the occurrence ofthe group delay and the more detailed cause analyzed in step S1204A arepresented to the user via display unit 110.

In step S1203, a determination is made whether the acquired individualdelay determination result indicates error (x). When YES, the processingproceeds to step S1204B, and when NO, the processing proceeds to stepS1205.

In step S1204B, since the individual delay determination resultindicates error, it is found that a delay occurs only in terminal device801 (terminal device A2 in the example of FIG. 11A). Therefore, a moredetailed cause is analyzed (details are identical to those in stepS1204A). After step S1204B, the error determination processing proceedsto step S1206, and the occurrence of the individual delay and the moredetailed cause analyzed in step S1204B are presented to the user viadisplay unit 110.

In a case where no error occurs in either of steps S1202 and S1203, instep S1205, a screen on which various pieces of information extractedfrom the received response data are arranged is created and is presentedto the user via display unit 110.

[1-3. Effects and the Like]

As described above, the network management system according to the firstexemplary embodiment simultaneously makes the group delay determinationand the individual delay determination on terminal devices 801 managedin the group. As a result, when a connection failure occurs betweennetwork management device 804 and terminal device 801, the user canspecify whether the connection failure is derived from terminal device801 or networks 803A, 803B, and take an appropriate measure.

Other Exemplary Embodiments

In the first exemplary embodiment, networks 803A, 803B are treated asgroups A, B, respectively. However, the method for dividing into aplurality of groups is not limited thereto, and for example, a pluralityof networks may be included in one group, or a plurality of terminaldevices connected to one network may be classified into a plurality ofgroups.

Further, in the first exemplary embodiment, only terminal device 801 ismanaged. However, the management target is not limited to the terminaldevice, and may be, for example, any device that can be connected to anetwork such as a router device and return response data fortransmission data.

As described above, the exemplary embodiments have been described asexamples of the technique of the present disclosure. For that purpose,the accompanying drawings and the detailed description have beenprovided.

Therefore, in order to illustrate the above techniques, the componentsdescribed in the accompanying drawings and the detailed description caninclude not only components essential to solve a problem but alsocomponents essential to solve a problem. For this reason, it should notbe immediately construed that those unessential components are essentialonly based on the fact that those unessential components are describedin the accompanying drawings and the detailed description.

In addition, the above-described exemplary embodiments are intended toillustrate the technique in the present disclosure, and thereforevarious changes, replacements, additions, omissions, and the like may bemade within the scope of the claims and equivalents thereof.

The present disclosure is applicable to a network management system fora network including a plurality of terminal devices.

What is claimed is:
 1. A network management device that divides aplurality of terminal devices connected to a network into a plurality ofgroups, transmits transmission data to the plurality of terminaldevices, and receives response data corresponding to the transmissiondata to acquire connection states of the plurality of terminal devices,the network management device comprising at least one of: an individualerror determiner configured to determine that an individual error occursin a target terminal device belonging to a group among the plurality ofgroups when a difference between (i) a response time from a transmissiontime of the transmission data to a reception time of the response datafor the target terminal device and (ii) an average response time that isan average value of the response time for each of two or more terminaldevices belonging to the group is larger than a first predeterminedvalue; and a group error determiner configured to determine that a grouperror occurs in a target group to which two or more terminal devicesbelongs among the plurality of groups when a difference between (i) acurrent average response time that is an average value of the responsetime for each of the two or more terminal devices and (ii) a previousaverage response time that is an average value of a previous responsetime for each of the two or more terminal devices is larger than asecond predetermined value, the previous response time being immediatelyprior to the response time.
 2. The network management device accordingto claim 1, further comprising a time information output unit configuredto output a current time, wherein the individual error determiner or thegroup error determiner acquires a time when the transmission data istransmitted and a time when the response data is received from the timeinformation output unit, and measures a difference between the time whenthe transmission data is transmitted and the time when the response datais received as the response time.
 3. The network management deviceaccording to claim 1, wherein the network management device presets areference response time for the plurality of terminal devices, and theindividual error determiner determines that the individual error occurswhen a difference between the reference response time and the responsetime of the target terminal device is larger than a third predeterminedvalue.
 4. The network management device according to claim 1, whereinthe network management device presets a reference average response timefor the plurality of groups, and the group error determiner determinesthat the group error occurs when a difference between the referenceaverage response time and the current average response time is largerthan a fourth predetermined value.
 5. The network management deviceaccording to claim 1, further comprising a display unit configured todisplay the acquired connection states including the individual error orthe group error.
 6. A network management system comprising: theplurality of terminal devices; and the network management deviceaccording to claim
 1. 7. A network management method for dividing aplurality of terminal devices connected to a network into a plurality ofgroups, transmitting transmission data to the plurality of terminaldevices, and receiving response data corresponding to the transmissiondata to acquire connection states of the plurality of terminal devices,the network management method comprising: measuring, for each of theplurality of terminal devices, a response time from a transmission timeof the transmission data to a reception time of the response data; andat least one of determining that an individual error occurs in a targetterminal device belonging to a group among the plurality of groups whena difference between (i) the response time for the target terminaldevice and (ii) an average response time that is an average value of theresponse time for each of two or more terminal devices belonging to thegroup is larger than a first predetermined value; and determining that agroup error occurs in a target group to which two or more terminaldevices belongs among the plurality of groups when a difference between(i) a current average response time that is an average value of theresponse time for each of the two or more terminal devices and (ii) aprevious average response time that is an average value of a previousresponse time for each of the two or more terminal devices is largerthan a second predetermined value, the previous response time beingimmediately prior to the response time.